A direct methanol fuel cell includes an anode to which an aqueous methanol solution is fed as a fuel, a cathode to which an oxidant is fed, and a proton conducting membrane placed between the anode and the cathode. The anode and the cathode each have a current collector for electron conduction and a catalyst layer formed on the current collector. The proton conducting membrane is placed between the respective catalyst layers. Each catalyst layer is made of a mixture of a catalyst and a perfluoroalkylsulfonic acid polymer such as Nafion (trade name, manufactured by DuPont). The perfluoroalkylsulfonic acid polymer in each catalyst layer has high chemical stability and functions to hold the catalyst. It is also used as a component resin to form a proton conducting membrane serving as an electrolyte membrane. Each catalyst layer also has catalyst fine particles such as platinum or platinum-ruthenium particles having a particle size of a few nanometers, supported on carbon particles, in which the oxidation or reduction of methanol or oxidant (e.g., air) is carried out.
However, since the perfluoroalkylsulfonic acid resin has a cluster structure in which hydrophilic groups are clustered together, it can swell with the aqueous methanol solution. The swelling of the perfluoroalkylsulfonic acid resin causes blocking of passages for methanol diffusion so that methanol is inhibited from diffusing to and reaching the catalyst surface. It also causes degradation of the catalyst. In addition, the three-phase interface formed between the perfluoroalkylsulfonic acid resin phase, the catalyst phase and the fuel phase (methanol phase) is destroyed by the swelling of the resin so that the proton conductivity at the three-phase interface is reduced. Thus, there is the problem of reduction in cell output power and cell life.